The present invention relates generally to a mechanical seal for the rotary shaft of a rotary machine for preventing the leakage of an internal liquid and more particularly a mechanical seal capable of attaining and maintaining reliable seal performance regardless of the positive and negative pressure difference between the internal and external liquids as encountered in compressors and pumps.
With the conventional stationary balanced type mechanical seals to be described in detail below, when the internal liquid pressure (the pressure of the liquid present internally of the mechanical seal) is higher than the exterior liquid pressure (the pressure of the liquid present externally of the mechanical seal), the satisfactory seal may be maintained, but when the external liquid pressure exceeds the internal liquid pressure, the force for forcing the seal face of a nonrotating ring to press against the mating seal face of a rotating ring mounted on the shaft becomes lower than the reaction due to the hydrodynamic pressure in film formed between the seal faces so that the seal faces are separated from each other and the leakage occurs.